Open Data supplied by Natural Environment Research Council (NERC)

PO311 Discrete CTD Nutrients Sampling Document

Originator's Protocol for Data Acquisition and Analysis

The FS Poseidon PO311 cruise departed on the 29 April 2004 from Santa Cruz de Tenerife, Spain and returned on the 24 May 2004. The cruise set out to test the hypothesis that: The supply of, and the interaction between iron and phosphorus control biological activity and fluxes in the subtropical North Atlantic.

Sample collection

Water samples were collected from a subset of CTD profiles. Samples were generally collected from 8 standard depths determined by light levels appropriate to production rate experiments.

Sample analysis

A total of 41 CTD casts were analysed for nutrients: nitrate, nitrite, phosphate and silicate using a Technicon AAII, five channel segmented flow autoanalyser within two hours of collection. Phosphate and silicate were determined according to (Kirkwood, 1989), nitrate and nitrite according to (Brewer and Riley, 1965) and (Grasshoff, 1976) respectively.

BODC Data Processing Procedures

Discrete nutrient samples were provided to BODC in Excel format. BODC extracted the data and merged with information from the associated Sea-Bird .btl files (also provided). These data were loaded to BODC's ocean database under the ORACLE Relational Database Management System. Data that were considered unrealistic were flagged suspect.

Samples in the database with a flag of "<" had concentrations below the specified detection limits. For nitrate and nitrite this was 0.01 µM and for phosphate this was 0.02 µM. There were no values below detection for silicate.

For any one experiment up to four CTDs were used to collect the water by targeting different depths for water collection. The original file only listed the possible CTDs for each experiment. BODC therefore matched as best as possible to the correct CTD and niskin number by using depth as a reference. Where duplicate depths were targeted, the bottle that fired first has been chosen.

For CTD cast 014, there was a large depth discrepancy. The Originator supplied a depth of 110 m and this has been linked to a firing depth of 93 m which was the closest firing depth.

No other discrepancies were found between the Originator's metadata and the metadata held centrally in the database.

Content of data series

The Originator's variables were mapped to appropriate BODC parameter codes as follows:

Originator's Parameter

Unit

Description

BODC Parameter code

BODC Unit

Comments

Nitrate + Nitrite

Micromoles per litre

Concentration of nitrate+nitrite {NO3+NO2} per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis

NTRZAATX

Micromoles per litre

No unit conversion necessary

Nitrite

Micromoles per litre

Concentration of nitrite {NO2} per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis

FeeP - Phosphate and Iron Addition Experiment

Who funded the programme?

The Natural Environment Research Council (NERC).

Who was involved in the programme?

The programme was a collaboration lead by Plymouth Marine Laboratory (PML) and involved scientists from the National Institute of Water and Atmospheric Research, New Zealand, Laboratoire Arago, France and the University of East Anglia.

What was the programme about?

The programme was a two ship exercise using RRS Charles Darwin and RV Pseidon to investigate the hypothesis that:

"The supply of, and interaction between, iron and phosphorus control biological activity and fluxes in the subtropical North Atlantic."

When was the programme active?

The project was carried out in 2004.

Brief summary of the programme fieldwork/data

An experimental area in international waters to the west of the Canary Islands was selected following an intense period of vertical and horizontal mapping by the Charles Darwin. Using SF6 as a tracer for amended waters, two separate experiments were performed: The first (5th - 15th May) involved the addition of 20 tonnes of anhydrous monosodium phosphate at 10 m depth over an area of approximately 25 km2, centered at 27.8 oN 23.3 oW. The second experiment was conducted following a mid-cruise return by the Poseidon to Tenerife between 16th May and 22nd May, at 27.5 oN 22.5 oW, when 5 tonnes of acidified iron sulphate were added over the first 12 hours, then following a brief recovery period, 20 tonnes of phosphate added over the top of the iron.

Measurements of nutrient chemistry, gas exchange and biological activity were monitored prior to and after deployment (IN stations) of the fertilised patches relative to several (OUT) control stations.

Please note:the supplied parameters may not have been sampled from all the bottle firings described in the table above. Cross-match the Sample Reference Number above against the SAMPRFNM value in the data file to identify the relevant metadata.